1. Field of the Invention
This invention relates generally to methods and apparatus for measuring conditions downhole in a well drilling operation, and particularly to a method and apparatus for combining a downhole measurement with a related measurement at the surface of a well.
2. Background Art
Downhole conditions can be measured at high sample rates, but the data cannot be transmitted uphole rapidly while drilling. These measured conditions are typically transmitted by sending pressure pulses through the drilling mud which fills the drill string connecting the drill bit to the surface. Sending these pulses through the drilling mud provides only one transmission path, so data must be transmitted in serial fashion. Since this transmission method limits data rates to approximately several bits of data per second, and since transmitting a single downhole measurement to the surface requires a number of bits of data, it requires as much as several seconds of transmission time to send a measurement signal from downhole to the surface.
Also, there are numerous downhole conditions of interest to be measured in drilling a typical well. Serial transmission requires that each of these measurements must wait its turn to be transmitted.
In addition to being limited to a single, serial data path for transmitting numerous measurements, there is also a limit to the speed of transmission along the data path. It typically requires 2 to 3 seconds for a signal to travel from downhole, up through the mud in the drill string, and to the surface. Although a downhole condition may be sampled much more frequently by downhole measurement devices, because of these other limitations, in many applications the measurement of a single downhole condition might be updated at the surface only about once every 30 to 60 seconds.
For a variety of reasons it is desirable to overcome the above described constraints to obtaining a rapid indication of the downhole effect of a surface condition. A drilling record with frequent updates may be useful after drilling for interpreting results of the drilling operation. Also, an operator needs downhole information in order to make timely adjustments in controlling the drilling process so that changing conditions can be detected and analyzed, such as changes in the friction between the drill string and the wellbore, the condition of the drill bit, and the lithology of the formation. These adjustments are important in order to maximize the rate of penetration and to drill safely, thereby minimizing expensive drilling time.